The invention relates to a power transmission unit with an input member and an output member and a hydraulic coupling dependent on a rotational-speed difference, in which, when a rotational-speed difference occurs between the input member and the output member, a hydrostatic displacement machine produces in a pressure space a pressure that acts on a piston acting on a friction clutch, the friction clutch having first and second disks connected in terms of drive to the input member and the output member respectively, and one of the members forming a housing that contains the displacement machine.
Power transmission units of this kind are used especially in drive trains of motor vehicles, preferably all-wheel-drive vehicles; either together with a differential, the hydraulic coupling limiting the differential action, or to drive the second driven axle, the torque transmitted depending on the difference between the wheel speed and the drive shaft connected to the wheels of the other axle. The pressure produced by the displacement machine acts on a clutch, preferably a multi-plate clutch. This action can be influenced by means of various valves, whether these are automatically acting valves or valves actuated by an external control system.
U.S. Pat. No. 5,536,215 has disclosed a power transmission unit of this kind, as has Austrian Utility Model 2964. In these and all such power transmission units, the pressure space in which the pressure acting on the piston is built up is in the rotating housing. As a result, the operating fluid contained in this housing is subject to a centrifugal force, which increases and thus distorts the pressure prevailing in the pressure chamber as a function of the rotational speed. This is particularly disruptive if the pressure is dependent on a rotational-speed difference and is supplied by a hydrostatic displacement machine, and this applies in both possible cases: if, in the first case—that of an unregulated coupling—there are no control valves, compensation is impossible; and if, in the second case, control valves intended to depressurize the pressure space for disengagement are provided, this is not possible at higher absolute rotational speeds because the discharge line adjoining the control valve has to end in a smaller radius. However, the pressure there is always less than in the pressure chamber, owing to centrifugal force.
It is therefore the object of the invention to eliminate these disadvantages corresponding to the special features of couplings of the generic type. The influence of centrifugal force should be at least partially compensated for to a necessary extent.